It's pretty simple it means your program is handling non-numeric key. Try dis with your program ./caesar abaed You're program doesn't treat this as an error because the keys are supposed to be only numbers, no other characters but numbers only.
Have you tried your programme? After finishing the programme, your prompt is on the same line as the output, but should be on the next line.
Add a printf (or equivalent) printing just a newline \n, place it after your for loop. but before you return.